It's just barley credible that it has a smaller, shuttle sized counterpart ("Pre-emptive Strike") that there is also a cruiser sized variant AND that someone welded it to an Intrepid saucer and it is still somehow a functioning starship just pushes it over edge for me.
Actually, there'd be three sizes: The shuttle-sized "Preemptive Strike" version, Chakotay's raider, and then this even larger version slapped to the Intrepid saucer. Scale-wise, Chakotay's ship would still be too small against the saucer.
Of course, we could sit here all day trying to make excuses about the design, but the truth is that it was just never meant to be taken seriously. That's why it was in the far background. Someone probably built it as a practical joke, an Easter Egg, or a "Where's Waldo" type deal. If we concede this ship's existence as a true Starfleet vessel, then we must also concede that Starfleet had two ships shaped coincidentally like the Millenium Falcon during the Sector 001 battle in FC, or that Starfleet had a decommissioned ship in the Qualor II surplus depot that coincidentally looked like a mothership from the "V" miniseries.
Rev: It looks like we're going to have to agree to disagree about the Renaissance class.